FIG. 1 schematically shows prior art image recording apparatus 100 with light-emitting diode (LED) print-head 101. Full width array imagers used in image recording systems are generally comprised of a linear array of discrete sources. The sources may emit ink, ions, or light. Examples of full width array imagers include wire dot, electrostatic, ink jet, and thermal print heads. Print-head 101 is an example of an LED full width array imager. An LED full width array imager consists of an arrangement of a large number of closely spaced LEDs in a linear array. By providing relative motion between the LED printbar and a photoreceptor in a process direction, and by selectively energizing the LEDs at the proper times in a scan direction, a desired latent electrostatic image can be produced on the recording member. The production of a desired latent image is usually performed by having each LED expose a corresponding pixel on the recording member in accordance with image-defining video data information applied to the printbar through driver circuitry. Conventionally, digital data signals from a data source, which may be a Raster Input Scanner (RIS), a computer, a word processor or some other source of digitized image data is clocked into a shift register. Some time after the start of a line signal, individual LED drive circuits are then selectively energized to control the on/off timing of currents flowing through the LEDs. The LEDs selectively turn on and off at fixed intervals to form a line exposure pattern on the surface of the photoreceptor. A complete image is formed by successive line exposures.
The following provides further detail regarding prior art apparatus 100. Print-head 101 includes: LED's controlled according to recording signals supplied from an unrepresented external device; a rotary drum 102 provided with a photosensitive member along the periphery thereof; a rod lens array 103 for focusing the light beams of the LED's in the printing head 101 onto the photosensitive surface of the drum 102; a corona charger 104 for charging the photosensitive member in advance; a developing station 105 for developing an electrostatic latent image with toner; a recording sheet 106; a cassette 107 housing a plurality of recording sheets 106; a feed roller 108 for feeding the recording sheet 106 from the cassette 107; registration rollers 109 for matching the front end of the recording sheet with the leading end of the image formed on the drum 102; a transfer charger 110 for transferring the developed image from the drum 102 onto the recording sheet 106; a separating roller 111 for separating the recording sheet from the drum 102; a belt 112 for transporting the recording sheet; fixing rollers 113; discharge rollers 114 for discharging the recording sheet onto a tray 115; a blade cleaner 116 for removing the toner remaining on the drum 102; a container 117 for the recovered toner; and a lamp 118 for eliminating charge remaining on the drum 102.
The function of the above-described apparatus is as follows. Upon turning on of an unrepresented main switch, there are activated a motor for rotating the drum 102, the lamp 118 and the corona charger 104, thus eliminating the toner, charge and hysteresis remaining on the drum. Then a recording enable signal is released to the external device when the fixing rollers 113 reach a fixing temperature by means of an internal heater.
In response to recording information supplied from the external device, the LED's in the printing head 101 emit light beams which are guided to the drum 102 through the rod lens array 103. The charge formed on the drum 102 by the charger 104 is selectively eliminated, in the exposure position, by the light beams from the printing head 101, thus forming an electrostatic latent image on the drum. The latent image is rendered visible by toner deposition in the developing station 105, and the visible image thus obtained is transferred onto the recording sheet by means of the transfer charger 110. The recording sheet is supplied from the cassette 107 by the timed function of the feed roller 103, and passes through the image transfer position, by means of the registration rollers 109, at a speed same as the peripheral speed of the drum.
After the image transfer; the recording sheet is separated by the separating roller 111, then supplied by the belt 112 to the fixing rollers 113 for image fixation, and discharged by the roller 114 onto the tray 115. The drum surface after the image transfer is cleaned with the blade cleaner 116 and is exposed to the light from the lamp 118 for erasing the hysteresis.
Matrix drive is used with high resolution light-emitting diode (LED) print-heads to reduce power dissipation and the number of wire bonds, enabling such print-heads to be made smaller, cheaper and more easily, for example, as taught by U.S. Pat. No. 6,172,701. Additionally, technology has been developed that enables LEDs and drivers to be integrated onto one CMOS substrate further increasing size, cost and reliability, for example, as taught by the website: “http://www.oki.com/en/press/2006/z06085e.html”
FIG. 2A illustrates prior art 1200 dots per inch (dpi), scan direction, print-head 150 implementing a matrix drive with an integrated LED/Driver. FIG. 2B is a block diagram of chips from print-head 150 in FIG. 2A. Groups 156 of LEDs 152 are connected appropriately to enable “⅛” matrix drive (only one of eight LEDs are strobed at any time during printing). A single matrix driver 158 controls all the LEDs in each group 156. In the example, there is an array of 15360 LEDs 102 constructed from 40 individual LED/Driver chips 154, where within each LED/Driver chip the LEDs are sectioned into 48 groups of eight LEDs.
FIG. 2C shows a group 156 of LEDs for a chip in FIG. 2A. The LEDs within each group of LEDs are arranged at 1200 dpi in the scan direction and offset in the process direction by ⅛ of a 2400 dpi to enable 2400 dpi resolution in process direction P.
A prior art LED print-head, such as 101 or 150, is limited both by time and power constraints. For example, the print-head is constrained by the amount of image data that can be transferred to and accepted by the individual LEDs within a certain time period. If incomplete data is transferred and accepted, respective printing operations are degraded. In general, it is desirable to minimize this time period to optimize printing rates. However, at the same time, there must be sufficient time to discharge the individual LEDs on respective desired pixel areas while a recording sheet is moving in the process direction. If, in the interest of increasing printing rate, there is insufficient power or time to properly strobe the individual LEDs, printing quality suffers.
FIG. 3A is a prior art matrix drive timing chart for the group of LEDs shown in FIG. 2C. Print-head 100 is able to print at both 1200 dpi×2400 dpi (scan×process) and 1200 dpi×1200 dpi using “⅛” matrix drive timing as shown in FIG. 3A. However, as shown below, printing at 1200 dpi×1200 dpi results in poor performance.
FIG. 3B shows respective spot images 160 for LEDs 152 in FIG. 2C at 1200 dpi×2400 dpi resolution according to the chart of FIG. 3A. This figure shows acceptable alignment of spot images for 1200 dpi×2400 dpi printing.
FIG. 3C shows respective spot images 160 for LEDs 152 in FIG. 2C at 1200 dpi×1200 dpi resolution according to the chart of FIG. 3A. Because the offset of the LED arrangement (⅛ at 2400 dpi) does not match the strobe timing interval (⅛ at 1200 dpi), 1200 dpi×1200 dpi printing with “⅛” matrix drive results in a misalignment (spot alignment error) of 7D/8 for each group of LEDs. This misalignment results in degradation of print quality.
FIG. 4A is a prior art matrix drive timing chart. FIG. 4B shows respective spot images 160 for LEDs 152 in FIG. 2C at 1200 dpi×1200 dpi resolution according to the chart of FIG. 4A. FIGS. 4A and 4B illustrate a prior art approach for 1200 dpi×1200 dpi printing. To address the misalignment problem for 1200 dpi×1200 dpi printing shown in FIG. 3C, it is known to print at the process speed for 2400 dpi but ignore every other line. This eliminates the misalignment but significantly reduces print speed. As shown in FIG. 4A and 4B, it is possible to compress the strobe time from “⅛” to “ 1/16” of a 1200 dpi line to eliminate the error. However, such a compression falls prey to the time and power constraints noted supra. For example, such compression reduces the maximum power of LEDs 152 by at least 50% and, as a result, either doubles the print data rate or halves the print speed when compared to 2400 dpi, both of which are undesirable.